Prota 1: Cereals and pulses/Céréales et légumes secs
Repert. bot. syst. 1: 779 (1843).
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
2n = 22
Vigna sinensis (L.) Hassk. (1844).
– Cowpea, black-eye bean, black-eye pea, China pea, marble pea (En). Niébé, haricot à l’œil noir, pois yeux noirs, cornille, voème, haricot dolique, dolique mongette (Fr). Caupi, feijão frade, feijão da China, feijão miúdo, feijão macundi, makunde (Po). Mkunde (Sw).
– Yard-long bean, asparagus bean (En). Haricot-kilomètre, dolique asperge (Fr). Feijão de metro, feijão chicote, feijão espargo, feijão frade alfange (Po).
– Catjang cowpea, Bombay cowpea (En). Catjang (Fr).
Origin and geographic distribution
Vigna unguiculata originated in Africa, where a large genetic diversity of wild types occurs throughout the continent, southern Africa being richest. It has been introduced in Madagascar and other Indian Ocean islands, where it is sometimes found as an escape from cultivation. The greatest genetic diversity of cultivated cowpea is found in West Africa, in the savanna region of Burkina Faso, Ghana, Togo, Benin, Niger, Nigeria and Cameroon. Cowpea was probably brought to Europe around 300 BC and to India 200 BC. As a result of human selection in China, India and South-East Asia, cowpea underwent further diversification to produce two cultivar-groups, Sesquipedalis Group with long pods used as a vegetable, and Biflora Group grown for the pods, dry seeds and for fodder. Cowpea was probably introduced to tropical America in the 17th century by the Spanish and is widely grown in the United States, the Caribbean region and Brazil.
Cowpea is the most important pulse crop in the savanna regions of West and Central Africa, where it is also an important vegetable and a valuable source of fodder. In East and southern Africa it is also important both as a vegetable and a pulse. Only in humid Central Africa is it less prominent.
Cowpea is the preferred pulse in large parts of Africa. The mature seeds are cooked and eaten alone or together with vegetables, spices and often palm oil, to produce a thick bean soup, which accompanies the staple food (cassava, yam, plantain). In West Africa the seeds are decorticated and ground into a flour and mixed with chopped onions and spices and made into cakes which are either deep fried (‘akara balls’), or steamed (‘moin moin’). In Malawi the seeds are boiled with their seed coat, or the latter is removed by soaking and leaving the seeds in the soil for a few hours. Small quantities of cowpea flour are processed into crackers, composite flour and baby foods in Senegal, Ghana and Benin.
The leaves and the immature seeds and pods of cowpea are eaten as vegetables. Cowpea leaves are served boiled or fried and are usually eaten with a porridge. The leaf may be preserved by sun-drying or boiling and then sun-drying to be used during the dry season. Leaves to be preserved for later use are generally plucked towards the end of the season. It is believed that leaves developed towards the end of the season are tastier as they tend to grow under conditions of stress. In Botswana and Zimbabwe boiled cowpea leaves are kneaded to a pulp and squeezed into small balls, which are dried and stored. Immature, green and still soft seeds are cooked to a thick soup and used as relish. The tender seedless cowpea pods are sometimes used as a cooked vegetable, as are young pods of yard-long bean. In Asia this is the most important use of cowpea, in Africa it is uncommon. In Benue State, Nigeria, the stringless coiled pods with little parchment of a landrace called ‘Eje-O’Ha’ are parboiled for a few minutes, opened and split in half. The seeds are eaten directly while the pod walls are dried and preserved for later use. Pods are also eaten locally in Benin. The roots are sometimes eaten, e.g. in Ethiopia and Sudan.
Cowpea is used as fodder in West Africa, Asia (especially India) and Australia; it is used for grazing or cut and mixed with dry cereals for animal feed. In the United States and elsewhere cowpea is grown as a green manure and cover crop. In Nigeria special cultivars are grown for the fibre extracted from the peduncle after retting; the strong fibre is especially suitable for fishing gear, and produces a good-quality paper. The dry seeds have been used as coffee substitute.
Various medicinal uses of cowpea have been reported: leaves and seeds are applied as a poultice to treat swellings and skin infections, leaves are chewed to treat tooth ailments, powdered carbonized seeds are applied on insect stings, the root is used as an antidote for snakebites and to treat epilepsy, chest pain, constipation and dysmenorrhoea, and unspecified plant parts are used as a sedative in tachycardia and against various pains.
Production and international trade
According to FAO statistics the total annual world production of dry cowpea seeds in 1999–2003 was about 3.6 million t from 9.5 million ha. Other estimates indicate a higher production: over 4.5 million t from about 14 million ha. According to FAO 3.3 million t was produced annually in sub-Saharan Africa, from 9.3 million ha, mainly in West Africa (3 million t/year from 8.8 million ha), the main producers being Nigeria (2.2 million t/year from 5.1 million ha) and Niger (400,000 t/year from 3.3 million ha). Brazil, which is not included in the FAO cowpea statistics, is estimated to produce about 0.6–0.7 million t/year from 1.1–1.9 million ha. Cowpea seeds are produced for local consumption and surpluses are sold in local markets. International trade is mainly within West Africa, with the exporting countries in the drier Sahelian zone, and the importing countries in the more densely populated humid region along the coast. It has been estimated that at least 285,000 t was traded between West African countries in 1998, mainly from Niger to Nigeria, but the total trade is probably larger.
There are no statistical data on the quantity of leaves and pods harvested, but it is likely to be considerable. Fresh and dried leaves are much sold in urban markets and some are traded to neighbouring countries. Dried leaves in the form of black balls are exported from Zimbabwe to Botswana and South Africa. Yard-long bean is grown in Asia on hundreds of thousands of hectares, but is of minor importance in Africa.
The nutritional composition of leafy stem tips of cowpea per 100 g edible portion is: water 89.8 g, energy 121 kJ (29 kcal), protein 4.1 g, fat 0.3 g, carbohydrate 4.8 g, Ca 63 mg, Mg 43 mg, P 9 mg, Fe 1.9 mg, Zn 0.3 mg, vitamin A 712 IU, thiamin 0.35 mg, riboflavin 0.2 mg, niacin 1.1 mg, folate 101 μg, ascorbic acid 36 mg. Young cowpea pods with seeds contain per 100 g edible portion: water 86.0 g, energy 184 kJ (44 kcal), protein 3.3 g, fat 0.3 g, carbohydrate 9.5 g, Ca 65 mg, Mg 58 mg, P 65 mg, Fe 1.0 mg, Zn 0.3 mg, vitamin A 1600 IU, thiamin 0.15 mg, riboflavin 0.15 mg, niacin 1.2 mg, folate 53 μg, ascorbic acid 33 mg. Yard-long bean pods contain per 100 g edible portion: water 87.9 g, energy 197 kJ (47 kcal), protein 2.8 g, fat 0.4 g, carbohydrate 8.4 g, Ca 50 mg, Mg 44 mg, P 59 mg, Fe 0.5 mg, Zn 0.4 mg, vitamin A 865 IU, thiamin 0.1 mg, riboflavin 0.1 mg, niacin 0.4 mg, folate 62 μg, ascorbic acid 19 mg. Immature cowpea seeds contain per 100 g edible portion: water 77.2 g, energy 377 kJ (90 kcal), protein 3.0 g, fat 0.4 g, carbohydrate 18.9 g, fibre 5.0 g, Ca 126 mg, Mg 51 mg, P 53 mg, Fe 1.1 mg, Zn 1.0 mg, vitamin A 0 IU, thiamin 0.1 mg, riboflavin 0.15 mg, niacin 1.45 mg, folate 168 μg, ascorbic acid 2.5 mg. Mature cowpea seeds contain per 100 g edible portion: water 12.0 g, energy 1407 kJ (336 kcal), protein 23.5 g, fat 1.3 g, carbohydrate 60.0 g, fibre 10.6 g, Ca 110 mg, Mg 184 mg, P 424 mg, Fe 8.3 mg, Zn 3.4 mg, vitamin A 50 IU, thiamin 0.85 mg, riboflavin 0.23 mg, niacin 2.1 mg, vitamin B6 0.36 mg, folate 633 μg, ascorbic acid 1.5 mg. The essential amino-acid composition per 100 g mature, raw cowpea seeds is: tryptophan 290 mg, lysine 1591 mg, methionine 335 mg, phenylalanine 1373 mg, threonine 895 mg, valine 1121 mg, leucine 1802 mg and isoleucine 956 mg. The principal fatty acids are per 100 g edible portion: linoleic acid 343 mg, palmitic acid 254 mg, linolenic acid 199 mg and oleic acid 88 mg (USDA, 2004). The approximate fatty acid composition of fat from cowpea seeds is: saturated fatty acids 25%, mono-unsaturated fatty acids 8%, polyunsaturated fatty acids 42%. Cowpea protein is relatively rich in lysine, but poor in S-containing amino acids. Cowpea seed is lower in antinutritional components such as lectins and trypsin inhibitors than common bean (Phaseolus vulgaris L.), and is easier and quicker to cook.
Adulterations and substitutes
The pods of common bean are often used for the same dishes as yard-long bean, although the taste is not the same. Immature seeds of several leguminous plants are used as substitutes for immature cowpea seeds, e.g. those of pea (Pisum sativum L.), common bean and lima bean (Phaseolus lunatus L.).
Climbing, trailing or more or less erect annual or perennial herb, cultivated as an annual; taproot well developed, with many lateral and adventitious roots; stem up to 4 m long, angular or nearly cylindrical, slightly ribbed. Leaves alternate, 3-foliolate; stipules ovate, 0.5–2 cm long, spurred at base; petiole up to 15(–25) cm long, grooved above, swollen at base, rachis (0.5–)2.5–4.5(–6.5) cm long; stipels small; leaflets ovate or rhombic to lanceolate, (1.5–)7–14(–20) cm × (1–)4–10(–17) cm, basal ones asymmetrical, apical one symmetrical, entire, sometimes lobed, glabrous or slightly pubescent, 3-veined from the base. Inflorescence an axillary or terminal false raceme up to 35 cm long, with flowers clustered near the top; rachis tuberculate. Flowers bisexual, papilionaceous; pedicel 1–3 mm long, with spatulate, deciduous bracteoles; calyx campanulate, tube c. 5 mm long, lobes narrowly triangular, c. 5 mm long; corolla pink to purple, sometimes white or yellowish, standard very broadly obovate, hood-shaped, c. 2.5 cm long, wings obovate, c. 2 cm long, keel boat-shaped, c. 2 cm long; stamens 10, 9 fused and 1 free; ovary superior, c. 1.5 cm long, laterally compressed, style upturned, with fine hairs in upper part, stigma obliquely globular. Fruit a linear-cylindrical pod 8–30(–120) cm long, straight or slightly curved, with a short beak, glabrous or slightly pubescent, pale brown when ripe, 8– 30-seeded. Seeds oblong to almost globose, often laterally compressed, 0.5–1 cm long, black, brown, pink or white; hilum oblong, covered with a white tissue, with a blackish rim-like aril. Seedling with epigeal germination; cotyledons oblong or sickle-shaped, thick; first two leaves simple and opposite, subsequent leaves alternate, 3-foliolate.
Other botanical information
Vigna comprises about 80 species and occurs throughout the tropics. However, the tropical American species are likely to be placed in a separate genus in the near future, which would reduce the genus to 50–60 species.
Vigna unguiculata is extremely variable, both in wild and cultivated plants. Several subspecies (up to 10) have been distinguished, most of them comprising perennial wild types, but subsp. unguiculata includes annual wild types and cultivated ones.
In cultivated Vigna unguiculata 5 cultivar-groups are generally recognized, although the groups can be crossed readily and overlap:
– Unguiculata Group (common cowpea): pulse and vegetable types, grown for the dry or immature seeds, young pods or leaves; plant habit prostrate to erect, up to 80 cm tall, late flowering, pods 10–30 cm long, pendent, hard and firm, not inflated when young, many-seeded and seeds not spaced; most African cultivars belong to this group.
– Sesquipedalis Group (yard-long bean, synonyms: Dolichos sesquipedalis L., Vigna sesquipedalis (L.) Fruhw.): grown for the young pods; plant climbing, stem up to 4 m long, pods 30–120 cm long, pendent, inflated when young, many-seeded and seed spaced; important vegetable in South-East Asia, but of minor importance in tropical Africa, where only cultivars introduced from Asia are grown.
– Biflora Group (catjang cowpea): grown for the seeds, tender green pods and for fodder; plant habit prostrate to erect, up to 80 cm tall, early flowering, pods 7.5–12 cm long, erect or ascending, hard and firm, not inflated when young, few-seeded and seeds not spaced; important in India and South-East Asia, locally also in Africa (e.g. Ethiopia).
– Melanophthalmus Group: originating from West Africa; plant able to flower quickly from the first nodes under inductive conditions, pods comparatively few-seeded, seed coat thin, often wrinkled, partly white.
– Textilis Group: a small group only grown in Nigeria for the fibre extracted from the long peduncles; at the beginning of the 20th century this group was distributed from the interior delta of the Niger river eastward to the Lake Chad basin, but it is gradually disappearing.
In Africa there are numerous landraces and improved cultivars within Unguiculata Group. Leaves are traditionally picked in cowpea fields grown primarily for the dry seed and belong to the top ten most popular leafy vegetables in many African countries. In addition, special types with erect plant habit or prostrate stems with long tender shoots are grown as a leafy vegetable, sometimes also for the immature seeds or young pods. The use of dual purpose types (seeds and leaves) is becoming very popular in some countries as the leaves are the main vegetable during the early rainy season.
Various cultivars of yard-long bean are offered by Asian seed companies, with a large variation in plant characters.
Growth and development
Germination of cowpea takes 3–5 days at temperatures above 22°C. The optimum temperature for germination is about 35°C. Flowers open in the morning and close before noon; they fall the same day. In dry climates cowpea is almost entirely self-pollinated, but in areas with high air humidity cross-pollination by insects may amount to 40%. Only fairly large insects are heavy enough to open the keel. The length of the reproductive period is very variable, with the earliest cultivars taking 30 days from planting to flowering, and less than 60 days to mature seeds. When leaves are harvested during the early growth stages, senescence starts 1.5–2 months after sowing and the plant dies after 3–4 months, depending on crop health and intensity of harvesting. Late cultivars with indeterminate growth take 90–100 days to flower and up to 240 days for last pods to mature. Cowpea forms N-fixing nodules with Sinorhizobium fredii and several Bradyrhizobium species.
Wild types of Vigna unguiculata grow in savanna vegetation, often in disturbed localities or as a weed, up to 1500 m altitude, but some can be found in grassland subject to regular burning, sandy localities close to the coast, woodland, forest edges or swampy areas, occasionally up to 2500 m altitude.
Cowpea grows best at day temperatures of 25–35°C; night temperatures should not be less than 15ºC and consequently cultivation is restricted to low and medium altitudes. At altitudes above 700 m growth is retarded. Cowpea does not tolerate frost, and temperatures above 35°C cause flower and pod shedding. It performs best under full sunlight but tolerates some shade. Cowpea is generally grown as a rainfed crop in sub-Saharan Africa, but in Asia it is sometimes grown on residual moisture after an irrigated rice crop. Short-duration determinate types can be grown with less than 500 mm rainfall per year; in experiments in Senegal ‘Ein al Ghazal’ produced 2400 kg/ha of seeds with only 450 mm rain. Long-duration types require 600–1500 mm. Yard-long bean tolerates high rainfall; a fully-grown crop has a water requirement of 6–8 mm per day. Cultivation in the dry season with ample irrigation is practised, as well as cultivation during the rainy season, although sowing during the rainy season can result in damage to the emerging or young plants. Most cowpea cultivars are quantitative short-day plants, but day-neutral types also exist. Cowpea can be grown on a wide range of soil types with pH 5.5–6.5(–7.5), provided they are well drained. It is moderately sensitive to salinity and exhibits greater salt tolerance during later stages of growth.
Propagation and planting
Farmers normally use farm-saved seed for planting. The 1000-seed weight of cowpea is 150–300 g. The seed rate for pure stands is 15–30 kg/ha. Seed dressing with an insecticide and a fungicide (e.g. thiram) prior to planting is recommended. In tropical Africa cowpea is mostly grown intercropped or in relay with other crops such as yam, maize, cassava, groundnut, sorghum or pearl millet. Pure stands are not common except in the coastal areas of East Africa, and also in Asia and Western countries. In the forest and Guinea savanna zones of West Africa cowpea is mainly intercropped with maize, cassava, yam or groundnut, at a very low density (1000–5000 hills/ha). In the northern Guinea savanna zone cowpea is intercropped with groundnut and/or sorghum. The component crops are normally planted in rows with systematic intercropping patterns, which may vary from alternate row intercropping to within-row intercropping with varying distance, giving a grid of groundnut or sorghum rows crossed by the cowpea rows every 2–3 m. The cowpea population is low, with individual plants spread over a 2–3 m radius. In the Sudan savanna cowpea is intercropped with pearl millet, sorghum and/or groundnut, in diverse and complex traditional intercropping patterns with varying interplant distances and planting sequences of component crops. For instance, in some areas of Kano state in Nigeria (Minjibir and Gezawa areas) pearl millet is planted first in rows 1.5–3 m apart at the onset of the rains (May–June), with 1 m distance within the row, resulting in 4000–6000 hills/ha. When the rains become more stable towards the end of June, pulse-type early cowpea cultivars are planted between alternate pearl millet rows at a distance of 1 m. Fodder-type, late-maturing cowpea is planted later, in mid-July, in the remaining rows. When grown as a sole crop, cowpea is sown at densities ranging from 22,000 plants/ha for prostrate types to 100,000 plants/ha for erect types. Recommended planting distances for sole-cropped cowpea in Kenya are 60 cm between rows and 20 cm within the row. In Swaziland spacings are 50 cm between rows and 15 cm within the row for erect cultivars. For landraces the spacings are much wider, especially for the dual purpose types. Often 2–3 seeds are sown per pocket, with thinning afterwards, e.g. during weeding. The sowing depth is 4–5 cm. Cowpea requires soil with fine tilth for good root growth. Generally, deep ploughing followed by harrowing provides an adequate tilth. In intercropping systems, tillage normally follows the crop in which cowpea is interplanted.
Peri-urban vegetable farmers use special cultivars for ratoon cropping of the leaves. They broadcast the seed on raised beds, made on well-manured soil, aiming at a dense stand of about 25 plants per m2.
Farmers in Africa use yard-long bean seed harvested from a previous crop, in contrast to South-East Asia, where many farmers procure healthy seed from improved cultivars. The 1000-seed weight of yard-long bean is lower than that of cowpea, 100–150 g. Seed is sown in pockets of 2–4 seeds. Cultivation is usually on raised beds for good drainage and easy surface irrigation and for easy staking and harvesting. Earthing-up the young plants protects the shallow root system and gives support to the seedlings. Some farmers apply mulch of rice straw, but this is not a common practice.
Cowpea derives a significant amount of its nitrogen requirements from the atmosphere and may leave 75–150 kg/ha in the soil for the benefit of the succeeding crop. If cowpea is grown in localities where it has not been grown recently, inoculation with nitrogen-fixing bacteria has been found to be beneficial. Cowpea requires phosphorus for nodulation and root growth. Incorporation of 25 kg/ha P is adequate for plant growth in phosphorus-deficient soils. In soils known to be deficient in potassium, application of 25 kg/ha K is recommended. Cowpea must be kept weed free during the early stages of growth. Two to three weedings during the first 6 weeks after planting are recommended; once the crop is established it outcompetes weeds. Weeding is usually done by superficial hoeing.
Cowpea grown as a vegetable and yard-long bean have a high mineral uptake. In soils of average fertility an application is recommended of 5–10 t/ha of farmyard manure during soil preparation, together with N 20 kg/ha, K 25 kg/ha and P 40 kg/ha. Three weeks after emergence a top dressing of 50 kg/ha urea is given. In yard-long bean, 2–2.5 m long stakes are inserted near the seed beds before sowing or during the first two weeks after emergence, before the plants have reached a height of 30 cm. A cheap method of staking is to relay-plant yard-long bean next to the stems of maize before or just after the cobs are harvested.
Diseases and pests
Cowpea is susceptible to a wide range of diseases and pests. Yard-long bean suffers from the same diseases and pests as cowpea but seems less susceptible than cowpea under humid conditions. Fungal diseases are more troublesome during the rainy season, whereas insect and mite pests and virus diseases cause more damage during the dry season.
The major fungal diseases are anthracnose (Colletotrichum lindemuthianum), Ascochyta blight (Phoma exigua), brown blotch (Colletotrichum truncatum), leaf smut (Protomycopsis phaseoli), leaf spot (Cercospora canescens, Septoria vignae, Mycosphaerella cruenta synonym: Pseudocercospora cruenta), brown rust (Uromyces appendiculatus), scab (Elsinoë phaseoli), powdery mildew (Erysiphe polygoni), pythium soft stem rot (Pythium aphanidermatum), stem canker (Macrophomina phaseolina) and web blight (Thanatephorus cucumeris, synonym Rhizoctonia solani). Crop rotation and the use of chemicals and resistant cultivars are necessary for integrated disease control. Bacterial diseases include bacterial blight (Xanthomonas campestris pv. vignicola), which occurs worldwide, and bacterial pustules (Xanthomonas axonopodis pv. glycines synonym: Xanthomonas campestris pv. vignaeunguiculatae) reported from Nigeria. These bacteria are seed-transmitted and secondary spread occurs by wind-driven rain. Control measures include the use of pathogen-free seeds, seed treatment with a mixture of antibiotics and fungicides such as streptocycline plus captan, and strict crop rotation. Resistance genes are available for bacterial blight and bacterial pustules.
Many viruses attack Vigna unguiculata. Some viruses of economic importance are cowpea aphid-borne mosaic potyvirus (CABMV), cowpea mottle carmovirus (CPMoV), cowpea yellow mosaic virus (CYMV), black eye cowpea mosaic potyvirus or bean common mosaic potyvirus (BCMV), cucumber mosaic cucumovirus (CMV-CS) and cowpea golden mosaic virus (CPGMV). Some of the viruses are seedborne, while aphids, white flies and beetles perform field transmission. Control measures include use of healthy seed of resistant cultivars if available, and weeding to remove alternative hosts. In poor sandy soils, cowpea is attacked by root-knot nematodes (Meloidogyne spp.). It is also a host plant of, among others, reniform nematodes (Rotylenchus spp.), root-lesion nematodes (Pratylenchus spp.) and lance nematodes (Hoplolaimus spp.). Crop rotation and resistant cultivars are used to control nematodes.
Insect pests are also a major factor limiting cowpea production and may even cause total seed loss. In tropical Africa much damage is caused by cowpea aphids (Aphis craccivora), flower thrips (Megalurothrips sjostedti), legume pod borers (Maruca vitrata, Etiella zinckenella), pod bugs and seed suckers (e.g. Clavigralla tomentosicollis, synonym: Acanthomia tomentosicollis). Lygus beetle (Lygus hesperus), cowpea curculio (Chalcodermus aeneus) and green leafhoppers (Empoasca spp.) are of less importance. Yard-long bean is especially attractive to aphids (Myzus persicae, Aphis gossypii), green stink bug (Nezara viridula) and red spider mite (Tetranychus spp.); greasy cutworms (Agrotis ipsilon) often cause damage just after emergence. The bean shoot fly (Ophiomyia phaseoli) is a common pest; the larvae tunnel in the leaves and stems, and severely attacked young plants will die, whereas older plants will suffer from hampered growth and serious yield reduction. Lodging incidence is generally high in infested fields; tolerant cultivars may produce aerial roots above the wound. Another common pest is the bean pod fly (Melanagromyza sojae). The larvae damage the petioles and young pods. Control of insect pests involves protecting the seed with a systemic insecticide (e.g. carbofuran) at sowing or applied as a solution to the emerging seedlings in the planting holes. Plant debris and affected plants must be burned. Cowpea seeds are extremely vulnerable to storage pests, with the cosmopolitan cowpea weevil (Callosobruchus maculatus) being the major storage pest. Measures to reduce pest damage include application of inoffensive vegetable oil, neem (Azadirachta indica A.Juss.) oil or wood ash, roasting and bagging the seeds in airtight plastic bags, and storing as whole pods.
Use of chemicals, resistant cultivars, biological control and proper crop management such as intercropping and weeding are necessary for integrated pest management. Chemical control of insects is common practice on yard-long bean, but not on cowpea. Because of the risks for farmer and consumer (especially when leaves are harvested), these sprayings must be reduced to the strict minimum.
Two parasitic weeds are a serious problem: Alectra vogelii Benth. prevalent in the southern savanna regions of West Africa, East Africa and southern Africa, and Striga gesnerioides (Willd.) Vatke prevalent in the savanna regions of West and Central Africa. Crop rotation, deep cultivation, intercropping, early planting and use of resistant cultivars reduce infestation by these parasitic weeds.
Cowpea leaves are picked in a period from 4 weeks after emergence of the seedlings to the onset of flowering. In crops grown for the seed, farmers often harvest 10–20% of the leaves before the start of flowering with little detrimental effect on the seed yield. Stronger defoliation increasingly reduces flowering, fruiting and seed yield. Growers of leafy cowpea types cut the plants at about 10 cm above the ground for a succession of new shoots (ratooning). Green pods are harvested when the seed is still immature, 12–15 days after flowering. Harvesting of dry seed is done when at least two-thirds of the pods are dry and yellow. In indeterminate types harvesting is complicated by prolonged and uneven ripening; for some landraces harvesting may require 5–7 rounds. Mature seeds are usually harvested by hand. Sometimes plants are pulled out when most of the pods are mature. In the complex traditional intercrop patterns of Kano state (Nigeria), early cowpea and sorghum cultivars are harvested at the end of August or the beginning of September. The late cowpea and sorghum cultivars are harvested after the onset of the dry season, between October and November, when the leaves show signs of wilting. The fodder types are uprooted or cut from the base and rolled into bundles with the leaves intact. These bundles are then kept on roof tops or in tree forks for drying, and are used or sold in the peak dry season.
The first picking of yard-long bean pods in the desirable stage takes place 6–7 weeks after planting, depending on cultivar and market requirements. Normally the pods are picked when the outline of the seeds is just visible. Picking must be meticulous, because pods which are passed over until the next harvest will become tough and discoloured, with swollen seed, and may exhaust the plant. Successive harvests take place at least once a week (twice a week for a better tuned grading) during 4–8 weeks.
Farmers may harvest up to 400 kg/ha of cowpea leaves in a few rounds with no noticeable reduction of seed yields. In Nigeria climbing cultivars yielded 9–17 t/ha of fresh pods, whereas decumbent cultivars yielded 6–15 t/ha. The mean dry seed yield of the same cultivars was 1. 4–1.7 t/ha. The world average yield of dry cowpea seed is low, 240 kg/ha, and for fodder it is 500 kg/ha (air-dried leafy stems). Average yield of dry cowpea seeds under subsistence agriculture in tropical Africa is 100–500 kg/ha. The average seed yield in Niger is 120 kg/ha, in Nigeria 400 kg/ha, and in the United States 900 kg/ha. Apart from the effects of diseases and pests, the low yields are partly explained by the fact that the crop is mostly grown at low densities in intercropping systems, shaded by taller cereals. Furthermore, cowpea is often sown later in the rainy season, which results in a shorter crop duration due to photoperiod-sensitivity. A yield potential of 3 t/ha of seed and 4 t/ha of hay can be achieved in sole-cropping with good management. In the United States seed yields up to 7 t/ha have been obtained.
For yard-long bean, a total yield of 15 t/ha in a harvest period of at least one month is considered satisfactory, but yields as high as 30 t/ha have been reported.
Handling after harvest
Harvested leaves cannot be kept for long; they have to be sold within 2 days. The shoots can be kept longer by putting them in a basin with water. Cowpea leaves are frequently dried in the sun for preservation, either after boiling and squeezing to black balls, or directly as whole or broken leaves, or as powder. Green yard-long bean pods are tied in bundles of 20–40 and packed in baskets or crates for transport to the market. Yard-long bean is less susceptible to loss of weight by transpiration and to transport damage than most other vegetables. In cool storage (8ºC) the pods will keep for 4 weeks. Immature fresh cowpea seeds have a limited shelflife if stored at ambient temperatures, but at 8°C they can stay fresh for 8 days. In Europe, the United States and Japan, immature tender green pods are sometimes frozen or canned. As a pulse, the threshed seed should be dried thoroughly to a moisture content of 14% or less for good storability.
The International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria holds a collection of over 15,000 accessions of the cultivated cowpea and 1000 accessions of related wild Vigna; the University of California, Riverside, United States holds 5000 accessions. IITA characterized 8500 accessions for resistance to Maruca pod borer and sucking bugs, and 4000 for resistance to flower thrips, bruchids and viruses. The level of resistance to insect pests is high in the wild species Vigna vexillata (L.) A.Rich., especially to pod sucking bugs and Maruca pod borer. Many accessions of wild Vigna species possess high levels of resistance to the storage weevil.
Small collections of yard-long bean are present at the Asian Vegetable Research and Development Center (AVRDC), Shanhua, Taiwan and the Institute of Crop Germplasm Resources (CAAS), Beijing, China and in national institutes in Asia. Only very small collections of catjang cowpea exist. In Asia landraces of vegetable and pulse types of Vigna unguiculata are in danger of being lost since improved cultivars are widely grown. This process has also started in Africa.
Much work has been performed on Vigna unguiculata breeding, mostly for cultivars grown as a pulse, and in South-East Asia for yard-long bean. In the United States special cowpea cultivars for harvesting pods and young seeds have been developed. Selection criteria for cowpea concern resistances (to insect pests, diseases, nematodes, parasitic weeds, drought), plant type, seed type, yield and cropping system. IITA has a large breeding programme and distributes cowpea germplasm, breeding material and cultivars. In collaboration with the International Livestock Research Institute (ILRI), IITA initiated a breeding programme to develop improved cowpea cultivars that provide both seed for human consumption and fodder for livestock in the dry season. Improved cultivars have also been developed for intercropping. National programmes in many countries have released improved cowpea cultivars with resistances to bacterial blight, cowpea aphid-borne mosaic potyvirus, cowpea aphids, cowpea curculio, root-knot nematodes, cowpea weevil and parasitic weeds. New early maturing cultivars were developed for hot and dry conditions, e.g. ‘Ein al Ghazal’ and ‘Mouride’. Improved cultivars are often short, erect, determinate types selected for optimal dry seed production and less suitable for the traditional leaf picking. Wild African Vigna species have been successfully crossed with Vigna unguiculata.
Breeding work on African vegetable types is scarce. Simlaw Seeds in Kenya has commercialized ‘Kenduke-1’, a semi-trailing type selected for large leaves with an attractive green colour and good taste and that can be picked for a long time. In Senegal the leaf vegetable ‘Fuuta’ with a vegetative period of up to 50 days was selected. The Crop Breeding Institute in Harare, Zimbabwe, selected dual-purpose cultivars with high leaf and seed yield; the Zimbabwean cultivar ‘Chigwa’ is specially suited for use as a leaf vegetable because of late flowering. ‘Melakh’ is a dual-purpose cultivar bred for dry and fresh seed production in Senegal.
Breeding of improved cultivars of yard-long bean by backcrossing and pedigree selection has been performed in South-East Asia. Yield is strongly correlated with pod length and the number of pods per plant. Resistance to bean flies would be welcome but seems difficult to achieve. East-West Seed Company in Thailand selected cultivars adapted to a wide range of growing conditions, e.g. ‘Aba’, with early maturity (first harvest 45 days after sowing), high yield, greyish green pods 60–70 cm long, and excellent market quality.
Genetic linkage maps of cowpea have been constructed using RAPD, AFLP and RFLP; the linkage maps have been used to locate genes conferring resistance to Striga gesnerioides, several viruses and root-knot nematodes, as well as to locate quantitative trait loci (QTLs) for time to flowering, time to maturity, pod length, pod and seed weight, and resistance to aphids. Direct organogenesis of cowpea has been achieved using hypocotyl, epicotyl or cotyledon tissue. Regeneration of cowpea via somatic embryogenesis has been attempted, but callus failed to regenerate plants at an acceptable frequency. Genetic transformation has been proposed, e.g. to achieve resistance to pests by incorporating Bacillus thuringiensis (Bt) genes and α-amylase inhibitor genes, but a robust system for stable genetic transformation of cowpea is not yet available.
Cowpea serves as a cheap source of plant protein, especially in West Africa. It plays an important role in multiple cropping systems and is a major component of integrated crop/livestock systems in West Africa. Diseases and pests are the major constraints in cowpea production. Resistance breeding could be of utmost importance to overcome these constraints, with an increasingly important role for biotechnological tools. Future improvement also relies on the collection of landraces and their wild relatives and their incorporation into breeding programmes.
The prospects for vegetable cowpea in Africa are bright. Apart from traditional dual-purpose cowpea cultivars (harvested as pulse and for the leaves) there is a need for special vegetable types. As a leaf vegetable: dwarf plants with erect or prostrate habit, long vegetative period, tender shoots and leaves. For immature seed: dwarf plants with erect or prostrate, determinate habit. For fresh pods: pods about 15 cm long (replacing French bean in hot lowland regions).
As a fruit vegetable, it seems logical to replace cowpea by yard-long bean, because of its superior yield and quality. Asian cultivars should be tested on suitability for tropical African conditions because, if combined with market development, yard-long bean has the potential to become an excellent enrichment of the available vegetable assortment.
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Sources of illustration
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Correct citation of this article:
Madamba, R., Grubben, G.J.H., Asante, I.K. & Akromah, R., 2006. Vigna unguiculata (L.) Walp. In: Brink, M. & Belay, G. (Editors). PROTA 1: Cereals and pulses/Céréales et légumes secs. [CD-Rom]. PROTA, Wageningen, Netherlands.
wild and planted
1, inflorescence; 2, fruiting branch; 3, seed.
flowering plants in the field
flowering and fruiting plant
Sesquipedalis Group, plants on the market
hedge of Sesquipedalis Group
the stringless landrace ‘Eje-O’Ha’